The present invention relates to an automatic gain control (AGC) circuit employed in an amplitude modulation receiver system. More particularly, the present invention relates to an AGC circuit employed in an amplitude modulation receiver system receiving an amplitude-modulated (positive modulated) broadcast wave of a video signal, wherein the voltage representing a gray level included in a video detection output is utilized as an AGC voltage, the gray level existing intermediate between a black level and a white level.
Shown in FIG. 1 is a conventional AGC circuit of mean value type, which may be adapted for a receiver system receiving the amplitude-modulated broadcast wave of a normal video signal, the wave having the peak value (Ap-p) of a carrier wave. The AGC circuit is constructed so that the output of a video-intermediate frequency amplifier 10 is applied through a diode 11 to a transistor 12 so as to derive a video detection output from the emitter of the transistor, the video detection output is applied to a filter circuit formed of a resistor 13 and a capacitor 14, and then a d.c. voltage signal output from the filter circuit, as a bias signal, is applied to the video-IF amplifier 10 through an AGC amplifier 15. The reference numeral 16 represents a tuning circuit.
However, in the AGC circuit shown in FIG. 1, the mean value of the video signal may fluctuate in response to the brightness of a picture, even if the intensity of electric wave is constant, because the output voltage of the filter circuit corresponds to such mean value. It is apparent, therefore, that the illustrated AGC circuit has a disadvantage such that the AGC voltage may also fluctuate.
In addition, in order to avoid appearance of 60 Hz component of the video detection output in the AGC voltage, a time constant of the filter circuit must be selected to a relatively large value of the order of approximately 0.2 (sec.). This large value of the time constant is indication of the fact that the AGC thereof is less rapidly responsible than other AGC circuits such, for example, as a Keyed AGC circuit and a peak value AGC circuit of a negative modulated receiver system. Accordingly, it is also apparent that the prior art AGC circuit illustrated in FIG. 1 has a further disadvantage such that is cannot follow effectively the sudden fluctuation of electric field.